1. Field of the Invention
The present invention relates to crosslinking slow release systems. More particularly, the present invention relates to encapsulation of polymer crosslinking agents to provide a slow release system.
2. Background Art
Crosslinking agents (or more commonly named vulcanization agents) are widely used in several types of polymers (such as rubber) to enhance their mechanical properties. The crosslinking agents (or vulcanization agents) are selected based on the types of polymers. For example, rubber is often vulcanized with sulfur agents. Hard rubber is vulcanized with 30% to 50% of sulfur, and soft rubber typically includes less than 5% of sulfur. The vulcanized rubber is not sticky, does not harden with cold or soften with heat, is elastic, springs back into shape when deformed, is highly resistant to abrasion and to most chemicals, and is a good insulator against electricity and heat.
Chemically, vulcanization involves the formation of cross-linkages between the polymer chains. Using rubber as an example, vulcanization involves mixing sulfur and rubber (and sometimes with an organic accelerator, such as an aniline compound, to shorten the time or lower the heat necessary for vulcanization) and heating the mixture in molds under pressure. Vulcanization can also be accomplished with peroxides, gamma radiation, and several other organic compounds.
Vulcanization is also used in the “curing” of other materials. For example, vulcanization of polymer dissolved in asphalt can enhance the elastomeric properties of the asphalt for use in the paving and roofing industry. While vulcanization improves the properties of such polymer modified asphalt, it is important to control the rates of vulcanization. A typical vulcanization tends to have a fast reaction (initial burst) at the beginning followed by slow reactions during the subsequent, undefined long period of time. The initial burst and the subsequent reactions are not desirable because they tend to form over-vulcanization, leading to thermoset components commonly known in the asphalt industry as “gels.” Asphalts that have been over cross-linked are not useful because they no longer have manageable viscosities even with heating.
Similar problems may also occur with the vulcanization of other polymers. In addition, when these polymers and crosslinkers are pre-mixed for storage, it is necessary that the vulcanization be controlled, either by use of controllable crosslinkers or by storing the mixture at sub-ambient temperatures to avoid pre-vulcanization. Otherwise, pre-vulcanization would yield products with poor mechanical properties. Therefore, it is desirable to have methods and products to control the rates of vulcanization in a polymer-crosslinker system.